Printers which print information such as a desired character or image on a sheet-like printing medium such as a paper sheet or film are widely used as an information output apparatus in a word processor, personal computer, facsimile apparatus, and the like.
Various methods are known as the printing method of the printer. In recent years, an ink-jet method has particularly received a great deal of attention because the ink-jet method enables noncontact printing on a printing medium such as a paper sheet, easily achieves color printing, and generates little noise. In terms of low cost and easy downsizing, printers generally widely adopt a serial printing arrangement in which a printing head for discharging ink in accordance with desired print information is mounted on a carriage, and printing is performed by reciprocally scanning the printing head in a direction crossing to the feed direction of a printing medium such as a paper sheet.
Many ink-jet printers perform discharge called preliminary discharge irrelevant to printing because of the two following reasons.
First, an inferior discharge occurs when a volatile component (solvent) contained in ink evaporates from the distal end of the nozzle (ink discharging element) of the printing head and ink thickens along with the lapse of time when no printing is performed. In order to prevent such inferior discharge and degradation in printing quality caused by the inferior discharge, preliminary discharge is performed.
Second, the ink-jet printer generally periodically performs suction recovery operation in order to prevent an inferior discharge caused by evaporation of the ink solvent from the distal end of the nozzle. At this time, if nozzles for discharging inks in a plurality of colors are sucked by one cap in a printer having printing heads for discharging inks in a plurality of colors for color printing, sucked inks mix with each other within the cap, are attached to the orifice surfaces of the printing heads, and reversely sucked into the nozzle, resulting in color mixing. Color mixing of ink may also occur in cleaning (wiping) the discharge surface with a cleaning blade or the like.
To prevent printing with color-mixed ink, a method of performing preliminary discharge is widely employed. That is, color-mixed ink is removed by discharge irrelevant to printing.
Timings when preliminary discharge is performed are immediately before the start of printing and during printing. Immediately before the start of printing, preliminary discharge is performed for removing abnormal ink from the distal end of a nozzle, the nozzle is filled with normal ink, and then printing starts. During printing, a time when normal discharge is possible is calculated from conditions such as the temperature of the printing head and the temperature and humidity inside the printer. Preliminary discharge is periodically performed at a time interval equal to or shorter than the calculated time.
Recently, user demands for ink-jet printers grow more and more, and a higher image quality, higher speed, lower cost, and smaller size are required.
To increase the quality, downsizing of ink droplets to be discharged and the use of many ink colors are adopted. Downsizing of ink droplets to be discharged means downsizing of printing dots to be formed on a printing medium, which greatly contributes to reduction in graininess at a highlight portion in a natural image. As for the use of many ink colors, a conventional general ink-jet printer forms an image with four, black (Bk), cyan (C), magenta (M), and yellow (Y) inks. To increase the image quality at a highlight portion and medium-density portion, there is proposed a printer using six inks including light cyan (LC) and light magenta (LM) inks of light tones prepared by decreasing the dye concentration.
To increase the speed, the number of nozzles per color and the driving frequency are increased.
Owing to increases in the numbers of colors and nozzles, the total number of nozzles used for printing greatly increases in comparison with a conventional printer. An arrangement capable of simultaneously discharging ink from all nozzles (full-color full discharge) requires a power supply unit capable of instantaneously supplying a large current to the printing head.
However, the use of such power supply unit is disadvantageous in terms of the cost and size; it becomes difficult to meet user demands for lower cost and smaller size.
Printing is performed by full-color full discharge only upon reception of a special pattern such as solid printing in 1-pass printing. Printing by full-color full discharge occurs very rarely in general printing operation.
From this, a simple, compact, low-cost power supply unit which cannot supply a current necessary for printing by full-color full discharge is mounted. In printing, the number of simultaneously driven nozzles (simultaneous discharge count) is counted. If the count exceeds a simultaneous discharge count corresponding to a current suppliable by the mounted power supply unit, the printer is so controlled as to switch the printing method such that the number of printing passes is increased.
As the driving frequency and the number of nozzles increase, the ink amount supplied from the ink tank to the printing head per unit time also increases. In general, the ink amount suppliable from the ink tank to the printing head per unit time is limited by the mechanical structure. If an ink amount exceeding the limit is supplied, ink is not normally supplied but contains bubbles, resulting in an inferior discharge.
To increase the ink amount suppliable from the ink tank to the printing head, the ink tank and supply channel must be upsized. This leads to high cost and large size, and it also becomes difficult to meet user demands.
In terms of the ink amount suppliable from the ink tank, the driving frequency may be restricted in discharge from all nozzles for each color (single-color full discharge).
For these reasons, it is often difficult to perform preliminary discharge by discharging full-color inks at the maximum driving frequency of the printing head.
In performing preliminary discharge, the driving frequency is set to one at which single-color full discharge is possible. A discharge color is limited, and preliminary discharge is performed a predetermined number of times for the color. After that, the preliminary discharge color is switched to sequentially perform preliminary discharge (sequential preliminary discharge).
However, this sequential preliminary discharge suffers the following problems.
Sequential preliminary discharge is performed for each color without simultaneously performing preliminary discharge for all colors. The time taken from the start to the end of preliminary discharge becomes long.
Color-mixed ink as a result of suction recovery operation or wiping operation on the discharge surface spreads from the nozzle into the liquid chamber. If the time till the start of preliminary discharge becomes long, color-mixed ink spreads into the liquid chamber. Color mixing cannot then be canceled unless a large amount of ink is removed.
That is, in sequential preliminary discharge of performing preliminary discharge for each color, the removal ink amount (preliminary discharge count) must be increased for some inks because such inks wait a longer time than other inks until preliminary discharge actually starts. An increase in ink amount consumed by preliminary discharge leads to an increase in the running cost of the printer